This invention relates, in general, to the formation of a thin semiconductor layer or membrane, and more particularly, to the use of an electrochemical etch-stop for forming semiconductor membranes.
Semiconductor devices such as piezoresistive pressure sensors and accelerometers utilize a thin semiconductor layer, or membrane, as a sensing or active element. A piezoresistor is formed in the membrane, and the electrical signal of the piezoresistor varies with the deformation of the membrane. Thus, accurate reproducibility and control of the membrane thickness is necessary to improve device performance.
A method used in the past to form such membranes entails using a potassium hydroxide solution to etch a cavity in a portion of an N-type silicon substrate. The portion of the substrate not etched is masked off by a silicon nitride layer. The potassium hydroxide anisotropically etches the unmasked portion of the substrate. The substrate is etched until a desirable thickness of the membrane is achieved. However, one disadvantage of this process is that an unacceptable variation in the membrane thickness results due to the thickness across a substrate varying by approximately 2.5 microns. This variation is unacceptable for membranes of thicknesses less than 25 microns, because the variation across the wafer is high in comparison to the thickness of the membrane.
Thus, it would be desirable to to be able to form thin membranes more uniformly and accurately. Thin membranes are desirable for use in accelerometer devices. Accelerometers measure vibration or acceleration, therefore require a thin membrane in order to produce a signal large enough to measure. Thinner membranes are also desirable for pressure sensors in order to be able to reduce the membrane size and thus the pressure sensor die size.
Another method of forming a membrane entails the electrochemical etching of a reverse biased P-N junction. The P-N junction consists of an N-type epitaxial layer formed on a P-type substrate. This method is thoroughly discussed in an article by Ben Kloeck et al, entitled "A Novel Four Electrode Electrochemical Etch-Stop Method for Silicon Membrane Formation," published in Tranducers '87, Japan, pp. 116-119. The reverse biased P-N junction provides a large etching selectivity of P-type silicon over N-type silicon in anisotropic etchants. The etching terminates at the P-N junction, thus the thickness of the membrane is determined by the thickness of the N-type epitaxial layer. This N-type epitaxial layer still has a plus or minus 10% variation in thickness across the wafer. Thus, further improvement in the membrane thickness would be desirable. In addition, the cost of forming an epitaxial layer is approximately twice as much as the cost of a plain substrate wafer. Furthermore, if active elements are on the wafer with the membrane, the N-type epitaxial layer thickness, and thus the membrane thickness, is limited to a maximum of 15 microns because the maximum depth of a diffused P.sup.+ isolation region in an N-type epitaxial layer is approximately 15 microns. Thus, it would be desirable to be able to fabricate thin membranes on semiconductor substrates without using an epitaxial layer.
Accordingly, it is an object of the present invention to provide an improved method of forming a thin semiconductor layer or membrane.
Another object of the present invention is to provide an improved method of forming thin semiconductor membranes having a small variation in thickness across a substrate wafer.
A further object of the present invention is to provide a method of forming an improved accelerometer device.
Yet another object of the present invention is to provide a method of accurately forming a thin membrane in order to be able to shrink device die sizes.